This invention relates to the field of cardiac resuscitation, and in particular to devices for assisting rescuers in performing cardio-pulmonary resuscitation (CPR). CPR is used to mechanically support circulation in subjects with cardiac arrest. Although, the American Heart Association (AHA) has proposed guidelines for CPR, the effectiveness of this intervention is difficult to actively assess as it is performed. The ZOLL Medical AED Plus system provides rescuers with valuable feedback on compression rate (metronome) and depth (audible prompts) to promote the proper CPR methodology.